The invention relates to scanners for writing image information onto, e.g., a photosensitive drum in the form of a light beam in image forming apparatuses such as laser printers and digital electronic copying machines. More particularly, the invention is directed to a means for eliminating a gap present at a portion where a rotating polygon mirror and a rotating shaft member are fitted together.
An image forming apparatus such as a laser printer and a digital electronic copying machine is designed to drive light-emitting means such as LEDs by image information fed from an image information output unit such as a computer and image information in the form of a light beam obtained by scanning a document. The image forming apparatus scans a surface of an image carrying body, such as a photosensitive body, by a laser beam to write image information thereon. The image forming apparatus such as a laser printer also includes a toner image forming mechanism on the image carrying body such as a photosensitive drum, adopting an electrophotographic system similar to that used in ordinary electronic copying machines. The toner image forming mechanism is designed to form a toner image by irradiating the laser beam onto the photosensitive body to thereby form a latent electrostatic image thereon and cause toner to adhere to such latent electrostatic image. Known as a device for writing image information on the photosensitive drum by the laser beam is a scanner disclosed in, e.g., Japanese Patent Unexamined Publication No. 230269/1990. Such a conventional scanner employs a means for writing images on the photosensitive drum in the width direction thereof by deflecting a laser beam using a rotating polygon mirror.
The above-mentioned conventional scanner is such a scanner 1 as shown in FIG. 3. In the scanner 1, a laser beam outputted from a not shown light source such as a semiconductor laser is reflected by the surface of a rotating polygon mirror 11 disposed on a rotating machine 10. A photosensitive drum 9 is scanned in the width direction thereof by a laser beam 8 reflected by the rotation of the rotating polygon mirror 11. The rotating polygon mirror 11 is disposed inside an optical box 2 with the upper surface of the optical box 2 closed with a cover 5 and is designed to be driven by the rotating machine 10 at high speeds. The laser beam reflected by the surface of the rotating polygon mirror 11 forms an image on the surface of the photosensitive drum 9 by passing through an image forming lens group 6 and a glass member 7 provided at an end of the box.
In the scanner i shown in FIG. 3, a drive means such as disclosed in Japanese Patent Unexamined Publication No. 309066/1990 or the like is used to drive the rotating polygon mirror 11 at high speeds. The rotating machine 10 proposed in the above-mentioned conventional example is constructed as shown in FIG. 4. A frame of the drive means is formed of the optical box 2. In the optical box 2, a fixed shaft 15 is firmly held by a lower locking screw 16 and a rotating shaft member 20 is rotatably supported by the fixed shaft 15 through upper and lower bearings 19, 19a. On top of the fixed shaft 15 are an upper ring 17 and a pre-load spring 18 so that the bearing 19 can be held.
The rotating shaft member 20 rotatably supported by the fixed shaft 15 places the rotating polygon mirror 11 at an upper portion thereof and a rotor member 25 with a rotor magnet 30 at a lower portion thereof. The rotor magnet 30 disposed on the rotor member 25 and a stator coil 31 disposed on the optical box 2 constitute a motor 12. The rotor magnet 30 is bonded onto the lower surface of the rotor member 25. The stator coil 31 and a stator yoke 32 are placed in a grove 3 arranged on a base member 2a. A control circuit 35 is arranged for the stator coil 31. When the control circuit 35 has been switched (turned on and off) and when pulsed current has been supplied, an intense magnetic field is generated in the vertical direction (as viewed in FIG. 4) to drive the motor 12. The stator yoke 32 is designed to improve motor efficiency by changing the direction of the magnetic field applied from the stator coil 31 from downward to upward.
The drive means can adjust dynamic balance by providing an annular groove 26 on the upper surface of the rotor member 25 and mounting a counterweight 27 on the groove 26. When the number of revolutions of the drive motor 12 is 5000 to 10000, no dynamic balance adjustment is required as long as manufacturing accuracy of components involved in the radial direction of the rotor member 25 and the rotor magnet 30 is improved. However, in excess of 10000 rpm, faster operation of the rotating polygon mirror requires that vibration be reduced. To overcome this problem, the scanner must adjust dynamic balance accurately by providing a means in which an annular groove is arranged on the rotor member or the like and mounting a counterweight thereon, so that high scanning accuracy can be satisfied.
The drive motor biases the inner ring of the upper bearing 19 downward by providing not only the upper ring 17 on top of the fixed shaft 15, but also the pre-load spring 18 between the upper ring 17 and the bearing 19, in addition to the dynamic balance adjustment means. Further, for the lower bearing 19a a labyrinth seal is formed by a groove 4 provided in the base member 2a and a lower projecting member of the rotating shaft member 20, so that lubricating oil within the bearing 19a will not splash out. Between the rotor member 25 and the rotating polygon mirror 11 is a spring 23 to bias the rotating polygon mirror 11 toward an upper stepped portion 21 of the rotating shaft member 20 so that the rotating polygon mirror 11 together with the rotating shaft member 20 can rotate together.
The thus constructed rotating body, using ball bearings such as shown in FIG. 4, can be operated at the number of revolutions ranging from about 5000 to 15000. In contrast thereto, a bearing member using a kinetic pressure spindle (pneumatic bearing) dedicated to high speed rotation is suitable for the number of revolutions ranging from 20000 to 40000. Even using both types of bearings, problems are encountered at the fitted portion between the rotating shaft member 20 and the rotating polygon mirror 11.
a) If a spring 23 is interposed between the rotor member 25 and the rotating polygon mirror 11, the rotating polygon mirror fixing position is displaced at 10000 rpm or more, causing the rotating polygon mirror 11 to vibrate in some cases. PA1 b) If a fixing means such as a screw is used to fix the rotating polygon mirror 11 on the rotating shaft member 20, too strong a torque of the screw causes the reflecting surface of the rotating polygon mirror 11 to deform, thereby impairing the flatness of the reflecting mirror. Smaller torques cause the rotating polygon mirror fixing position to be displaced, thus causing the rotating polygon mirror 11 to vibrate. PA1 c) If the rotating polygon mirror is fixed on the rotating shaft member by thermal fit, the rotating polygon mirror fixing position is difficult to be displaced, but the reflecting surface of the rotating polygon mirror deforms to a large degree. PA1 d) If an adhesive is used, the adhesive strength (intermolecular force) of the adhesive is too large that the reflecting surface of the rotating polygon mirror deforms to a large degree. Such deformation is aggravated with a thinner rotating polygon mirror, which is a disadvantage.
In addition to the rotating polygon mirror fixing means such as shown in items a) and b), the following techniques have also been employed.
As shown in FIG. 4, to cause the rotating shaft member to support the rotating polygon mirror, a fitted portion 22 between the rotating shaft member 20 and the rotating polygon mirror 11 has a gap (fit gap) of about 5 to 50 .mu.m. Negative effects of this gap at the fitted portion 22 are not grave as long as the rotating polygon mirror is driven at low speeds. However, in the case of high speed operation in excess of 10000 rpm, the rotating polygon mirror fixing position is displaced in the radial direction by the gap due to centrifugal force, thermal deformation, thermal impact or the like of the rotating polygon mirror. Therefore, even if dynamic balance adjustments are made correctly on the rotating polygon mirror, the rotor member, and the like, such dynamic balance is destroyed when these members are assembled into a rotating body. Further, in the case of the fixing means using a screw or the like, strong torque is applied to the rotating polygon mirror, which produces errors on the reflecting surface of the rotating polygon mirror. This makes accurate scanning of image information difficult.
The invention has been made in view of the above problems related to the gap between the rotating shaft member and the rotating polygon mirror. Accordingly, an object of the invention is to provide a means for allowing no gap to be present at a fitted portion between the rotating shaft member and the rotating polygon mirror while using particulates to fill the fitted portion of the rotating polygon mirror and for preventing such gap from negatively affecting the operation of the scanner as a whole even when the rotating polygon mirror is rotated at high speeds. Another object of the invention is to provide a method of manufacturing such a means.